Method and apparatus for increasing the intrinsic viscosity of a polycondensate melt

Abstract

In a method and an apparatus for increasing the intrinsic viscosity of a polycondensate melt at negative pressure, the melt enters a chamber, in which a negative pressure of less than 20 mbar prevails, through a perforated plate or a screen having openings with a diameter of less than 0.5 mm. The melt passes through this chamber in free fall in thin threads and remains in a reservoir beneath the chamber for at least one minute. The melt is moved constantly in the reservoir, and discharged from the reservoir, by a helical mixing and discharge part.

The invention claimed is: 
     
       1. A method for increasing the intrinsic viscosity of a polycondensate melt under negative pressure, in which the melt enters a chamber via a connection opening and through a perforated plate or a screen with a plurality of openings with a diameter of less than 0.5 mm, in which chamber there is a pressure of less than 20 mbar, wherein the melt passes through said chamber in free fall in the form of thin threads, and wherein the thin threads are returned to a melt bath and the melt remains in a reservoir below the chamber, wherein the melt remains in the reservoir for at least one minute and the melt in the reservoir is moved continually by a mixing and discharge element under a pressure of less than 20 mbar and the mixing and discharge element is aligned in a horizontal position, wherein a height of the melt bath in the reservoir is selected to be so high that the mixing and discharge element in the reservoir is not completely covered by the melt and thereby the surface of the melt is repeatedly torn up by a rotational movement of the mixing and discharge element and repeatedly renewed, and the polycondensation started with the thin threads is continued in the melt bath by the dwelling period and continuous movement, and thus causes polymer chain growth and thereby increases the intrinsic viscosity further and the melt is discharged from the reservoir laterally by the mixing and discharge element. 
     
     
       2. The method as claimed in  claim 1 , wherein in the chamber there is a pressure of below 10 mbar. 
     
     
       3. The method as claimed in  claim 1 , wherein there is a dried gas or an inert gas in the chamber. 
     
     
       4. The method as claimed in  claim 1 , wherein the melt enters the chamber through openings with a diameter of between 0.05 and 0.5 mm. 
     
     
       5. The method as claimed in  claim 1 , wherein the dwelling time of the melt in the reservoir is at least 1 to 2 minutes. 
     
     
       6. The method as claimed in  claim 5 , wherein the dwelling time of the melt in the reservoir is up to twelve, fourteen or sixteen minutes. 
     
     
       7. The method as claimed in  claim 1 , wherein after the mixing and discharge element an in-line viscosity measurement of the melt is performed and the intrinsic viscosity of the melt is controlled by changing the negative pressure. 
     
     
       8. The method as claimed in  claim 1 , wherein the melt passes through at least two chambers consecutively. 
     
     
       9. The method as claimed in  claim 8 , wherein the melt between two chambers is kept at the required melt temperature by a temperature control device. 
     
     
       10. The method as claimed in  claim 1 , wherein the mixing and discharge element discharges the melt from the reservoir into a collecting line, and
 wherein the mixing and discharge element extends into the connecting line.